1. Field of the Invention
The present invention relates to an attitude control device used for working on an object such as a silicon wafer or a magnetic disk substrate of which high shape/size accuracy and high flatness of a finished surface are required, and to a precision machining apparatus equipped with the attitude control device. More particularly, the present invention relates to an attitude control device and a precision machining apparatus capable of controlling the attitude of an object mounted on the attitude control device with high accuracy according to grinding stages.
2. Background Art
There has recently been an increasing demand for reducing energy loss in next-generation power devices while reducing the size of the devices. An example of such a demand includes a demand for increasing the number of layers in a semiconductor multilayer structure for electronics purposes and increasing the packaging density of semiconductor devices. Examples of methods conceivable as measures to meet such a demand include a method for reducing the thickness of semiconductor wafers typified by a silicon wafer to an extremely small value, a working method which prevents dislocation and lattice strain in a worked surface and a portion below a worked surface, and a working method which reduces the surface roughness (Ra) to a value in a range from the subnanometer (nm) level to the nanometer (nm) level and reduces the flatness of a worked surface to a value in a range from the submicrometer (μm) level to the micrometer (μm) level or a lower range.
In the motor vehicle industry, an integrated bipolar transistor (IGBT) which is a power device for motor vehicles forms an essential system in inverter systems. A further improvement in marketability of hybrid vehicles achieved by improving the performance of an inverter using the IGBT and by reducing the size of the inverter is being expected. Reducing the thickness of the Si wafer constituting the IGBT to 50 to an extremely small value of about 150 μm, preferably 80 to 140 μm, more preferably 90 to 120 μm to reduce switching loss, steady loss and thermal loss is indispensable to improving the inverter. Further, an improvement in yield in a process step of forming electrodes on the semiconductor and an increase in the number of layers in the semiconductor multilayer structure can be achieved by forming a perfect surface with no dislocation and no lattice strain in a worked surface of a circular Si wafer having a diameter of 200 to 400 mm or in an internal portion in the vicinity of the worked surface and by reducing the surface roughness (Ra) to a value in a range from the subnanometer level to the nanometer level and the flatness to a value in a range from the submicrometer level to the micrometer level.
In ordinary cases under present circumstances, a multistep process including rough grinding using a diamond grinding wheel, lapping, etching and wet chemo-mechanical polishing (wet-CMP) using a loose abrasive is required for the above-described semiconductor working process. It is extremely difficult to obtain a perfect surface by the conventional working method using such process steps, since an oxide layer, dislocation and lattice strain are produced in the worked surface. Also, the flatness of a wafer worked by the conventional method is low and a break in the wafer may be caused during working or after electrode formation, which leads to a reduction in yield. Further, in the conventional working method, the difficulty in reducing the wafer thickness to an extremely small value is increased with the increase in wafer diameter to 200 mm, to 300 mm and to 400 mm. Studies are presently being conducted to reduce the thickness of a wafer having a diameter of 200 mm to the 100 μm level.
There has been a demand for developing a device (attitude control device) capable of accurately moving a rotary device supporting an object to be ground such as a silicon semiconductor wafer by an extremely small distance when the above-described super-precision working is performed on the Si semiconductor wafer. This demand is based on the fact that the accuracy with which the surface of an object to be ground is ground with a grinding wheel cannot be improved without suitably correcting a misalignment between the axes of the grinding wheel and the object to be ground which occurs during the grinding process in addition to initialization for aligning the grinding wheel and the object to be ground.
A method of using a servo motor, a method of using a piezoelectric actuator using a piezoelectric element, a method of using a super-magnetostrictive actuator and the like are generally being practiced as a means for moving or displacing an object through a comparatively small distance with accuracy. If the above-described attitude control device is operated by a servo motor, it is difficult to implement the attitude control device because of a limit to the output performance of the servo motor. It is extremely difficult to perform control with accuracy on the order of nanometers if a servomotor or any other device using rotational motion is used. In a case where the attitude control device is operated by an piezoelectric actuator using a piezoelectric element, there is a problem that the element itself may be broken if it is used to displace a large-load object such as a rotary device used for grinding.
In view of the above-described problems of the conventional art, the inventors of the present invention made a study of an apparatus for accurately moving a large-load object on the order of tons with high accuracy on the nanometer or angstrom level, and developed an elastically displaceable table finely displaceable without friction by considering that in the case of mounting a large-load working unit on the order of tons on the elastically displaceable table displaceable without friction, a displacement of the working unit in a direction perpendicular to the direction of gravity is not influenced by the weight of the mounted object. An invention using this elastically displaceable table is disclosed in JP Patent Publication (Kokai) No. 2001-265441 A. This invention is constituted by the elastically displaceable table displaceable without friction and a super-magnetostrictive actuator. The elastically displaceable table is displaced through deformation of a rod provided on the super-magnetostrictive actuator to perform positioning. The elastically displaceable table displaceable without friction is constituted by a frame and elastically displaceable supports. The frame is mounted on a base by means of the supports. The supports are formed of a material having impact resilience like a spring and having a strength high enough to support an object weight on the order of tons. A plurality of super-magnetostrictive actuators are attached to end surfaces of the frame. A magnetic field excited by application of a current based on a position command value acts on each super-magnetostrictive element to cause the element to expand.
The inventors of the present invention also discloses in JP Patent Publication (Kokai) No. 2002-127003 A an invention relating to an attitude control device (a precision machining apparatus having the attitude control device) incorporating a control system to compensate for a response delay due to a hysteresis characteristic of the above-described super-magnetostrictive element, and a super-magnetostrictive actuator capable of preventing a reduction in accuracy due to joule heat or eddy current produced from a coil of the super-magnetostrictive actuator. The control system that compensates for a response delay due to a hysteresis characteristic uses a hysteresis model expressed by a mathematical function (a straight-line function, a circular-arc function, a spline function or the like), preferably an inverse model based on a Preisach model, which is a model for prediction of a change in a magnetized state of a magnetic material. In this attitude control device, two plates are connected by one connection member in a state of being spaced apart from each other. A super-magnetostrictive actuator is provided at one side of the upper plate perpendicular to a side closer to the connection member. Two super-magnetostrictive actuators are provided below a side of the upper plate opposite from the side closer to the connection member. The connection member is formed of a material elastically deformable and having a strength high enough to support an object weight on the order of tons, as is the corresponding member in JP Patent Publication (Kokai) No. 2001-265441 A. In this invention, the motions of the three super-magnetostrictive actuators are controlled so as to correct a fine misalignment between a grinding wheel axis and a work axis in a three dimensional space.
Each of the fine positioning device disclosed in JP Patent Publication (Kokai) No. 2001-265441 A and the attitude control device disclosed in JP Patent Publication (Kokai) No. 2002-127003 A is capable of accurately controlling positioning of an object having a comparatively large weight (e.g., a rotary device for grinding with a grinding wheel) while supporting the object. However, the two super-magnetostrictive actuators and one support (or connection member) are interposed between the two plates in these devices, and restraint by the support (or the connection member) on the movement of one of the plates relative to the other cannot be avoided since the support (or the connection member) is an elastic member of high strength. That is, if the strength of this member is higher, the movement of the plate in a direction perpendicular to the axis is restrained more strongly. The freedom of movement of the plate on the moving side is limited by this restraint. Also, if the plate is moved only by the super-magnetostrictive actuators, there is a problem that heat produced from the super-magnetostrictive actuators affects other component parts of the positioning device (attitude control device) and damage to the other component parts or a reduction in attitude control accuracy may result.